An image sensor converts an optical image into an electrical signal. Image sensors may be largely classified into complementary metal-oxide-silicon (CMOS) image sensors and charge coupled device (CCD) image sensors. A CCD image sensor may generally have better noise and photosensitivity characteristics compared with the CMOS image sensor. However, the CCD image sensor may be difficult in relatively high integration environments, and has relatively higher power consumption. The CMOS image sensor has a simpler manufacturing process, relatively higher integration, and lower power consumption, compared with the CCD image sensor. As technology for manufacturing semiconductor devices has developed, technology for manufacturing the CMOS image sensor and resultant characteristics thereof are greatly improved. Research into the CMOS image sensor continues.
In general, a pixel of a CMOS image sensor includes photodiodes for receiving light and CMOS elements for controlling image signals input through the photodiodes. The photodiodes generate electron-hole pairs according to the wavelengths and the intensities of red light, green light and blue light input through color filters. An output signal varies depending on the amount of generated electrons. Accordingly, an image can be sensed electronically.
A CMOS image sensor includes a pixel region for performing photoelectric conversion, and a peripheral circuit region for detecting signals converted in the pixel region. The peripheral circuit region surrounds the pixel region. In a CMOS image sensor, photoresist is coated over a semiconductor device to form a micro lens. The slope of the bottom surface of the semiconductor substrate should be minimized to achieve planarization such that the photoresist is evenly coated over the entire surface of the semiconductor device.
In the related art, an over chemical-mechanical polishing (over CMP) process may be performed in the pixel region so that the undoped silicate glass (USG) is thinner than that in the peripheral region. As shown in example FIG. 1, after array etching, a difference in thickness between a center B and an edge A of the semiconductor substrate occurs. This difference in topology adds to the difficulty in adjusting the thickness and the radius of curvature of the micro lens during micro lens fabrication. These problems have an influence on the manufacturing process and the quality of the image produced by the sensors.